AWA: Academic Writing at Auckland

Abstract

How homing pigeons (Columba livida) are able to find their home location when displaced large distances has long baffled scientists. The geomagnetic field has been suggested as an ideal navigational tool and observations on pigeons have suggested they use the field for location determination and navigation. The following is an investigation of how proximity to an anomaly in magnetic field affects the release orientation vanishing bearings of homing pigeons. Pigeons were released from three sites (Te Pua School road, Whitehills road and Wainui road) near Swanson (West Auckland) at locations in and around the Auckland Junction Magnetic Anomaly. Pigeons showed site bias, at no site did they orient towards home direction, instead aligning parallel or perpendicular to the local magnetic field. At areas of steeper field slope, pigeons on average oriented to the same direction whereas at a low slope site their orientations were scattered. This study provides evidence for the Walker model but further research needs to be done to fully appreciate this phenomenon

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Introduction

In order for any animal to find a specific location in space they require two key pieces of information, vector coordinates for its current location and goal so that the most efficient course to between the two can be decided (Walker, 1998). How homing pigeons (Columba livida) are able to find their home location when displaced large distances has long baffled scientists and remains one of the long standing unanswered questions of science (Mora & Walker, 2009; Walker, 1998; R. Wiltschko, Schiffner, & Wiltschko, 2009; R. Wiltschko & Wiltschko, 2003). Observations of the magnetic effects on pigeons have lead to suggestions that pigeons use the magnetic field for location determination (Walker, 1998).

The geomagnetic field has been suggested as an ideal navigational tool as it is ubiquitous, stable over evolutionary time, varies systematically (Skiles, 1985) and can be used to determine latitude (varying in total intensity and inclination from north – south) (Mora & Walker, 2009). If an animal were able to sense these differences it would allow it to establish any location on a “navigational map” to determine its location (Mora & Walker, 2009; R. Wiltschko et al., 2009). The simplest mechanism for orientation and location determination is by means of a bicoordinate system to uniquely define a location or vectors relative to and external reference point or goal (Walker, 1998; R. Wiltschko & Nehmzow, 2005). There have been several studies indicating clear responses from other animals to variations in the magnetic field (W. Wiltschko & Wiltschko, 2005). There is however, little explanation and understanding of how (Walker, 1998), though the discovery of small magnetite particles in the beak skins of pigeons has been suggested to provide a sensory means (R. Wiltschko et al., 2009; Yorke, 1981).

It has been suggested in past years that anomalies in the magnetic field around a release site could serve to confuse or disorientate pigeons potentially causing them to set off in the wrong direction (Keeton, Larkin, & Windsor, 1974; R. Wiltschko et al., 2009). An anomaly in the earth’s geomagnetic field is defined as a location where the field is distorted (Dennis, Rayner, & Walker, 2007) and therefore could potentially give false information about magnetic location. Multiple studies have suggested that before a pigeon takes off at a release site they have already established where they are in relation to home (Walker, 1998; R. Wiltschko et al., 2009). Therefore it makes sense that they would take off (or vanish) in a homeward direction, however this has been found not to be the case at many sites with nearby magnetic anomalies (Dennis et al., 2007; Keeton et al., 1974; Mora & Walker, 2009; Walker, 1998). This phenomenon is known as release site bias and is defined as persistent deviations from home direction at release sites (Keeton et al., 1974). The following is an investigation of how proximity to an anomaly in magnetic field affects the release orientation of homing pigeons.

Methods

Data from present study collected at Wainui road/Silverdale site location over a one-hour period on 09 of August 2010. Conditions for the Wainui road were mild with south-westerly winds of around 15 knots; with 3-5 tenths cloud cover and temperature of 13^oC. Due to inappropriate weather on scheduled release dates for other sites, data from 2009 was used for the Whitehills road and Te Pua school road sites. Data for Whitehills road was collected on 19 March 2009 with north-westerly winds of around 15 knots, 3 tenths cloud cover and temperature of 22^oC. Data for the collection for Te Pua School road site was done on 17 March 2009 with easterly winds of around 5 knots, 1 tenth cloud cover and temperature of 22^oC.

Release subjects and site locality

Birds used for this study were supplied by a member of a local pigeon racing club housed in a loft a few kilometres out of Kumeu (West Auckland, New Zealand). Pigeons were released at three locations in and around what is known as the Auckland Junction Magnetic Anomaly, which arises from a deep-seated source approximately 1.6 km below the surface not correlated with any surface topography. Specific release sites were on road sides (easy accessibility) and were locations with minimal visual interference for researchers and distraction for the birds (for example not too busy, minimal trees or buildings found immediately nearby).

Release procedures

Birds were kept in crates, covered to minimise stress, for transportation to study site. Once the bird was released, it was visually followed by binoculars by multiple researchers until the bird could no longer be seen by all. Bearing the pigeon was travelling in at 1 minute was recorded along with a vanishing bearing; vanishing time and notes on the distance and height at which the pigeon was lost were noted. After data collection all observations were collected and those that were deemed the best (furthest, highest and longest (time) observations) for each bird were chosen for further analysis.

Data analysis

Data selected and sorted in excel depending on the quality of observations. If observations stated; landed in trees, were near and low or did not provide all relevant data, then they were not included in the analysis. Degrees magnetic data was converted to true bearings and were corrected for homeward direction. Once sorted and selected, data was entered into the software Oriana to produce circular plots. The data for the Wainui road location was relatively split on either side of the circle, it has been statistically analysed as unimodal but has been converted and displayed as bimodal it to better display the data and better the accuracy of the confidence intervals. Rayleigh test for uniformity was conducted to establish if bearings were evenly distributed or not and a Watson-Wheeler test to check for differences between mean orientation directions.   

Results

Orientation of birds observed at one minute did not differ statistically from vanishing bearings (p=>0.05), therefore only vanishing bearings are displayed here. On no occasion did the direction of home fall between the confidence intervals of the direction that the pigeons were travelling in (Fig. 1). There were significant differences between the mean directions of Whitehills road and Te Pua School road (p=0.00) and between Wainui road and Te Pua School (p=0.03) release site bearings when corrected for home. The home-corrected bearings of pigeons released at Whitehills and Te Pua School road however were very similar (p=0.96)

Figure 1: Home corrected (home=0^o) vanishing bearings for pigeons released at three sites Te Pua School road, Whitehills road and Wainui road respectively, near Swanson in West Auckland (mean direction ± 95% confidence intervals).

Figure two shows vanishing bearings of pigeons at the three release sites. The pigeons released at Te Pua School road beared at an average vector of 200^o (r=0.61), Whitehills road pigeons averagely beared at 135^o (r=0.61) and Wainui road pigeons on averaged beared at 149^o (r=0.29).  Whitehills road and Te Pua School road sites showed a uniform distribution (p=0.01 and p=0.02 respectively) whereas the vanishing bearings for Wainui road were scattered and non-uniform (p=0.32). Vanishing bearing directions at Wainui road did not differ statistically from either Whitehills road or Te Pua School road (p=0.72 and p=0.23 respectively). Whitehills road vanishing bearings did however differ significantly from Te Pua School road release sites (p=0.02).

Discussion

It has been suggested by many in the literature that pigeons use the magnetic field for navigation (Dennis et al., 2007; R. Wiltschko & Nehmzow, 2005; W. Wiltschko & Wiltschko, 2005). At locations on the earth where there are small-scale distortions or anomalies in the field this can affect an organisms perception of location and orientation (Dennis et al., 2007; Skiles, 1985). It is evident that pigeons released here at sites in proximity to the Auckland Junction Magnetic Anomaly, are exhibiting site bias and not immediately orienting to the direction of home but are affected by some other factor (Fig. 1).

Walker (1998) proposed a model for navigation in pigeons suggesting the use of intensity and slope direction of the magnetic field as vector coordinates to establish location and relation to home. Results from studies have shown that at release sites near anomalies, pigeons will align themselves to the contours of the steepest slope on the magnetic field (Dennis et al., 2007; Mora & Walker, 2009; Walker, 1998). This behaviour is observed in the current study at the Whitehills road site where the contours are close together indicating a steep slope (Fig. 2). The Walker model also proposes that pigeons will fly perpendicular to contours to minimise the difference to home intensity (Walker, 1998), heading toward a known “home intensity”(R. Wiltschko & Nehmzow, 2005), which can then be followed until they find the right inclination that indicates home (like a house on a hill) (Walker, 1998). The loft is situated on a gradient contour of the anomaly. This perpendicular flight can be observed at the Te Pua School site (Fig. 2). This behaviour was also found by Dennis et al (2007) (parallel to and perpendicular flight to intensity contours) in a study conducted at the same anomaly. Due to the nature of an anomaly, the field around it is distorted (Dennis et al., 2007). Therefore, when pigeons are following a contour they believe they are flying in a different direction to that they are actually travelling in. 

It is posited under the walker model that homing is more accurate along contours of the steepest slope (Walker, 1998). This makes sense as this would require the least amount of flying involved to search for and establish changes in magnetic intensity (Walker, 1998; R. Wiltschko et al., 2009). It has been suggested that even though pigeons can sense the immediate total magnetic field, they are unable to predict the direction of slope (Walker, 1998; R. Wiltschko & Wiltschko, 2003). This means they are constantly required to sample the surrounding field in order to follow it in the right direction (Dennis et al., 2007; Walker, 1998). A location of gentle and low slope in the magnetic field would cover more area, therefore potentially requiring greater sampling effort before a detectable change in the magnetic intensity and would therefore require greater sampling effort, whereas changes in field intensity would be condensed and therefore easier to find and follow on a steep slope. Evidence of this can be observed when comparing the observations of Whitehills road and Wainui road (Fig. 2). The field slope is steep at Whitehills and very low at Wainui road. Most of the pigeons at Whitehills consistently set off in the same direction, which appears to coincide with the contours of the magnetic field (Fig. 2). Whereas, at Wainui road there did not appear to be a consensus on the direction that birds were heading. This non-directional behaviour is consistent with the searching hypothesis (Dennis et al., 2007; Walker, 1998) where the low slope of the contour requires a greater area to be searched to establish location.

For this study Te Pua School road and Wainui Road can be considered to be controls as they both lie outside the major contours of the Auckland Junction Magnetic Anomaly (Fig. 2). However despite this pigeons released at these sites behaved very differently in relation to home (Fig. 1) and the magnetic field (Fig. 2). Whilst at Wainui road search behaviour is apparent, Te Pua School road exhibited a uniform distribution and birds flew in a different direction to the Wainui birds. The Te Pua school road released birds flew perpendicular to the field contour lines. It is possible that this observation is them flying across the contours in the attempt of locating the same intensity as home to follow (Keeton et al., 1974). Though the slope at the release site is not as steep as observed elsewhere in this study site there is apparently enough slope for the pigeons to establish perpendicular course. It has been suggested that pigeons have the means to sense small changes in magnetic field intensity with high precision to establish location and orientation (Keeton et al., 1974; Walker, 1998; Yorke, 1981) there is little understanding of the extent to which this sense affects orientation and navigation (R. Wiltschko & Wiltschko, 2003). Studies have also suggested that there are varying levels to which individual birds are able to sense the field (R. Wiltschko et al., 2009).

It is entirely possible of course that observations made here are due to chance, influenced by sampling bias as a result of the small sample size and difficulties of working with live animals (don’t always do what want or expect). Though this study provides evidence for such ideas as the effects of magnetic anomalies of site bias (Keeton et al., 1974; Mora & Walker, 2009; Walker, 1998) and for the Walker model for pigeon navigation (Walker, 1998), there have been similar studies conducted where different results have been found and interpreted as providing evidence against such theories (R. Wiltschko et al., 2009; R. Wiltschko & Wiltschko, 2003). The results obtained and used for analysis are very subjective, based on estimated terms (such a near, far, high and low), which would have differed for each researcher depending on how they perceive each measure. As much data was removed due to inappropriate observations (involving trees, too short vanishing time), the sample size was small leading to a potential sampling bias. This could be rectified by increasing the number of birds released. It could also be interesting to look at how pigeons react to locations at different bearings to the loft, as the release sites in this study were to the north.

This study provides behavioural evidence for the Walker (1998) model proposing that pigeons used the magnetic field to determine location and navigate to home (goal). There is evidence that site bias observed in pigeon vanishing bearings, aligns with (either parallel or perpendicular to) the magnetic field. Why this is the case is still not entirely understood despite multiple studies and theories that attempt to describe and explain this phenomenon. More research needs to be conducted looking at pigeon sensitivity to the magnetic field to establish exactly how homing over long distances is possible in these remarkable birds.

Acknowledgements

I would just like to formerly thank Mike Walker for the awesome BIOSCI 337 course and his help and advice in the field and in the lab with this assignment. Also thanks to the suppliers of the pigeons used (and of course the pigeons) for their cooperation and Josh Guilbert for putting up with and answering my persistent questions.

No pigeons were harmed in the process of this research.

References

Dennis, T. E., Rayner, M. J., & Walker, M. M. (2007). Evidence that pigeons orientate to geomagnetic intensity during homing. Proceedings of the Royal Society of Biology.

Keeton, W. T., Larkin, T. S., & Windsor, D. M. (1974). Normal fluctuations in the earth's magnetic field influence pigeon orientation. Journal of Comparative Physiology, 95, 95-103.

Mora, C. V., & Walker, M. M. (2009). Do release site biases reflect response to the Earth's magnetic field during position determination by homing pigeons? Proceedings of the Royal Society of Biology, 276, 3295-3302.

Skiles, D. D. (1985). The geomagnetic field: its nature, history and biological relevance. In J. L. Kirshvink, D. S. Jones & B. J. McFadden (Eds.), Magnetite Biomineralization and Magnetoreception in Organisms (pp. 43-102). New York: Plenum Press.

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Wiltschko, R., Schiffner, I., & Wiltschko, W. (2009). A strong magnetic anomaly affects pigeon navigation. The Journal of Experimental Biology, 212, 2983-2990.

Wiltschko, R., & Wiltschko, W. (2003). Orientation behaviour of homing pigeons at the Gernsheim anomaly. Behaviourlal Ecology and Sociobiology, 54, 562-572.

Wiltschko, W., & Wiltschko, R. (2005). Magnetic orientation and mechanoreception in birds and other animals. Journal of Comparative Physiology, 191, 675-693.

Yorke, E. D. (1981). Sensitivity of pigeons to small magnetic field variations. Journal of Theoretical Biology, 89, 533-537.